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NANOTECHNOLOGY 2
EDITORS
Prof Dr. Mustafa ERSÖZ
Dr. Mine SULAK
Dr. Massimo BERSANI
Dr. Arzum IŞITAN
Meltem BALABAN
Dr. Zeha YAKAR
Dr. Cumhur Gökhan ÜNLÜ
Dr. Volkan ONAR
Denizli 2018~ 2 ~
NANOTECHNOLOGY 2
EDITORS
Prof Dr. Mustafa ERSÖZ
Dr. Mine SULAK
Dr. Massimo BERSANI
Dr. Arzum IŞITAN
Meltem BALABAN
Dr. Zeha YAKAR
Dr. Cumhur Gökhan ÜNLÜ
Dr. Volkan ONAR
(0258. 296 41 37 [email protected])
ISBN 978-975-6992-78-4
1st Edition – October 2018~ 3 ~
This book is an output of “Universal Nanotechnology Skills
Crea-tion and MotivaCrea-tion Development) / UNINANO” as numbered
2016-1-TR01-KA203-034520 supported by Turkish National Agency
under Erasmus+ Key Action 2 Strategic Partnership in the field of
Higher Education (KA203).
“Funded by the Erasmus+ Program of the European Union.
However, European Commission and Turkish National Agency
can-not be held responsi-ble for any use which may be made of the
in-formation contained therein”
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CONTENTS
PREFACE 7
UNINANO PROJECT 8
SECTION 1 INTRODUCTION TO NANOMATERIALS 9
1.1 NANOMATERIAL CHARACTERIZATION 11
1.1.1 Atomic Force Microscopy [AFM] 13 1.1.2 Auger Electron Spectroscopy [AES] 13 1.1.3 Fourier-Transform Infrared Microscopy (FITR) 13
1.1.4 Helium Ion Microscopy [HIM] 13
1.1.5 Dynamic Secondary Ion Mass Spectrometry [SIMS] 14 1.1.6 X-ray Fluorescence Analysis [XRF, EDX] 14 1.1.7 Grazing-incidence X-ray Fluorescence 14 1.1.8 Electron Backscatter Diffraction [EBSD] 15 1.1.9 Scanning Electron Microscopy [SEM] 15 1.1.10 Scanning Tunneling Microscopy [STM] 15 1.1.11 Static Secondary Mass Ion Spectrometry [S-SIMS] 15 1.1.12 Surface Raman Spectroscopy 16 1.1.13 Transmission Electron Microscopy [TEM] 16 1.1.14 X-ray Diffraction and Reflection [XRD] 16 1.1.15 X-ray Photoelectron Spectroscopy [XPS] 16 1.1.16 X-ray Reflectometry [XRR] 17
SECTION 2 MICROSCOPY 19
2.1 SEM ANALYSIS 21
2.1.1 Instrumentation 23
2.1.2 Application Cases 26
2.2 SCANNING PROBE MICROSCOPES (SPM) 34
2.2.1 Instrumentation 35
2.3 HELIUM ION MICROSCOPY (HIM) 49
2.3.1 Principles 50
2.3.2 Instrumentation 52
2.3.3 Application Nanostructured Ge Layers 53
SECTION 3 SPECTROSCOBY VE SPECTROMETRY 59
3.1 X-RAY DIFFRACTION (XRD) 61
3.1.1 Applications 63
3.2 X-RAY FLUORESCENCE ANALYSIS 67 3.2.2 X-Ray Fluorescence Analysis 72 3.2.3 Total reflection XRF, Grazing Incidence-XRF, 75
3.2.4 Instrumentation 77
3.2.5 Application Cases 79
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3.3.1 Principle of The Technique and Instrumentation 89
3.3.2 Application Cases 94
3.4 RAMAN SPECTROSCOPY 100
3.4.1. Classical Wave Interpretation 100 3.4.2 Quantum Particle Interpretation 103
3.4.3 Instrumentation 105
3.4.5 Example of Raman Spectra Analysis 106
3.4.6 Case study 107
3.5 SECONDARY ION MASS SPECTROMETRY (SIMS) 112
3.5.1 Basic Principles 115
3.5.2 SIMS Analytical Modes 117
3.5.3 Depth profiling 119
3.5.4 Applications 122
SECTION 4 APPLICATIONS 129
4.1 INTRODUCTION to SURFACE PLASMONS AND ITS APPLI 131 4.1.1 Surface Plasmon Polaritrons 132 4.1.2 Surface Plasmons Excitation 137 4.1.3 Surface Plasmons for Chemical and Bio Sensing 138 4.1.4 Plasmonic Photodetectors 140
4.2 ELECTRONICS APPLICATIONS 144
4.2.1 Nanoelectronics 145
4.2.3 Nanoelectronics in Communication Systems 153 4.2.4 Nanoelectronics in Medicine 153 4.2.5 Research & Development Areas in Nanoelectronics 154 4.3 APPLICATIONS of NANOBIOTECHNOLOGY 157 4.3.1 Use of Nanomaterials in Diagnostic Applications 158 4.3.3 Use of Nanomaterials in Implant and Prosthesis Applications 169
4.4 TEXTILE APPLICATIONS 177
4.4.1 Smart Textiles Produced with Nanotechnology 178 4.4.2 Nano Textile Production Methods 182 4.4.3 Use of Nanotechnology during Fiber and Yarn Production 183 4.4.4 Nano Finishing Processes 184
4.5 ENVIRONMENTAL APPLICATIONS 194 4.5.1 Use of Nanoparticles 194 4.5.2 Sustainable Products 195 4.5.3 Sensor Applications 201 4.6 MILITARY APPLICATIONS 206 4.6.1 Soldier Nanotechnologies 206 4.6.2 Bio Chemical Sensing, Health Monitoring, 212 4.6.3 Tracking, Tracing and Remote Identification 213
4.7 PACKAGING APPLICATIONS 218
4.7.1 Packaging 218
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4.7.3 Nanotechnology Packaging Design Strategy 221 4.7.4 Packages of the future 223 4.7.5 Appplication of Nano-Materials in Packaging 228
SECTION 5 INTERNATIONALNORMS and REGULATIONS 233
5.1 INTERNATIONAL NORMS AND REGULATIONS 235 5.1.2 What are the regulations for nanotechnologies? 236 5.1.3 ISO/TC 229 on Nanotechnologies 237 5.1.4 ISO/TC 229 on Nanotechnologies Objectives 238 5.1.6 Nanotechnology Norms Needs Issues 242
SECTION 6 NANOTECHNOLOGY and INNOVATION 249
6.1 INNOVATION in NANOTECHNOLOGY 251
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PREFACE
Nanotechnology, which is the fundamental technology of the industrial revolu-tion of 21st century, is the science of controlling matter at atomic and molecular levels. At its simplest meaning and depending on scientific determinations and experiences, as a consequence of its contribution to environment, energy, mate-rials strength and proper consumption, the share of nanotechnology in preser-ving the world’s livability is very clear.
Today, the high value-added technology is vital for business lines that require intense competition such as military, medical, automotive, textile applications. In recent years, nanotechnological investigations have brought a significant progress in especially materials science and many new products or process ta-king place in our lives..
In general, nanotechnology education is conducted in post-graduate level and the number of nanotechnology education programs within master’s and doctoral programs increase constantly in many Universities. However, nanotechnology education is very limited at undergraduate level in many natural sciences and engineering programmes.
The books aimed at natural sciences and engineering undergaraduate students as well as young students provide a complete review of all relevant aspects from the nanotechnology and applications perspectives. The books provide practice-based knowledge at undergraduate level through creating awareness of this subject area and also support visual and e-learning in degree schemes that rela-te to nanorela-technology marela-terials.
The Book 1 is devoted to provide a theoretical description of the basic principles and fundamental properties of nanotechnology.
The Book 2 is devoted to presenting the characterisation techniques, micros-copy, spectroscopy and application of nanotechnology for environmental, health and safety issues.
We would like to thank very much to all researchers and authors who contribu-ted to this two parts. We are deeply grateful to Erasmus+ Programme for fun-ding the Universal Nanotechnology Skills Creation and Motivation Develop-ment” KA203- Strategic Partnerships Project; 2016-1-TR01-KA203-034520 “ and the publication of these books.
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UNINANO PROJECT
You are reading Nanotechnology 2 book which is the one of the outputs of “Uni-versal Nanotechnology Skills Creation and Motivation Development / UNINA-NO” Project as numbered 2016-1-TR01-KA203-034520 supported by Turkish National Agency under Erasmus+ Key Action 2 Strategic Partnership in the field of Higher Education (KA203).
In UNINANO Project, Pamukkale University as coordinator and beneficiary institution, Selçuk University and Afyon Kocatepe University from Turkey, Bru-no Kessler Foundation and Cosvitec from Italy, Cluj-Napoca University from Romania, and CCS from Greece have taken part.
To increase awareness of nanotechnology which is one of Turkey's 2023 strate-gic goals has been the main objective of UNINANO Project. In line with this main objective, written and visual educational materials have been prepared, and aimed to contribute to the advancement of nanotechnology knowledge by students and instructors using these materials. For this purpose, two course books have been prepared in both printed and electronic versions, in both Tur-kish and English:
Nanotechnology 1: Fundamentals of Nanotechnology Nanotechnology 2: Characterization and Applications
The electronic versions of the books are available on the www.pau.edu.tr/uninano
project website. Additionally, the answers of the questions at the end of the book, also located on the web page can be accessed from e-learning materials.
With the happiness of completing our project;
We would like to thank to the Presidency of Turkey's National Agency for sup-port of our project.
We would like to thank to Rector of the Pamukkale University and Project Ma-nager Prof. Dr. Hüseyin BAĞ for his valuable support during two years.
We would like to thank to Prof. Dr. Mustafa Ersöz, Dr. Mine Sulak, and Dr. Massimo Bersani who worked scientific editoralship of the book, and Meltem Balaban who worked in the book chapters' organization and book chapter aut-horing. As well as, we would like to thank to Dr. Zeha Yakar, Dr. Cumhur Gök-han Ünlü, and Dr. Volkan Onar, the other project team members of Pamukkale University. In addition, we would like to thank to Dr. Yasemin Öztekin for her valuable support for typsetting.
For their valuable effort and authoring, we would like to thank to all authors: Dr. Arzu Yakar from Afyon Kocatepe University; Dr. Gratiela Dana Boca from Cluj-Napoca University; Dr. Mustafa Ersöz, Dr. Gülşin Arslan, Dr. Serpil Edebali, and Dr. İmren Hatay Patır from Selçuk University; Dr. Massimo Bersani, Dr. Mario Barozzi, Dr. Erica Iacob, Dr. Giancarlo Pepponi, Dr. Lia Emanuela Vanzetti, Dr. Rocco Carcione, and Dr. Giovanni Paternoster from Bruno Kessler Foundation. We would like to thank to Ali Gökçe who prepared the UNINANO logo, Aydın Uçar who prepared the cover design of the book, Can Kaya who helped in the book's typographic,and the students of Pamukkale University Technology Fa-culty who contributed to the project activities and meetings together with.
Dr. Arzum Işıtan Project Coordinator www.pau.edu.tr/uninano https://www.facebook.com/UninanoPAU/ https://instagram.com/uninano_pau https://twitter.com/Uninano_PAU
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SECTION 1
INTRODUCTION
TO
NANOMATERIALS
CHARACTERIZATION
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1.2 NANOMATERIAL CHARACTERIZATION
Massimo BERSANI
[email protected]
FONDAZIONE BRUNO KESSLER
If you don’t “see” you cannot do it
INTRODUCTION
The characterization is a key point for the nanomaterial development from the aspects of basic research to production activity. If nanomaterials and related technologies have to achieve an effective accuracy level and efficiency a dedica-te application of analytical dedica-techniques (nanometrology) cannot be overlooked. The nanometrology have to allow a complete material characterization regarding chemical and physical aspects, electrical and structural proprieties, thermal and tribological characteristics etc. with a spatial resolution in three dimension aro-und a nanometer or below. To aro-understand nanomaterial characteristics and beha-viors it is mandatory to develop and upgrade the analytical instrumentation and related methodology. The basic research, the fundamental mechanisms unders-tanding the applications development and industrial production monitoring requ-ire a powerful and complete analytical approach. The terrific development poin-ted out by microelectronic technology demonstrapoin-ted the mandatory and indis-pensable factors. In fact the microelectronic development has been characterized from the beginning by the metrological support. Without the impact of dedicated analytical techniques and specific methodologies the microelectronic era certa-inly did not have the advances and the impact on our lives that we know today. In the nanotechnology field the impact of analytical techniques will be, if it is possible, more important.
In Figure 1.1.1 is reported a general underlying scheme of analytical techniques. A well-defined probe is used to induced a local input in the samples. The sample feedback is the emission of various signal from a specific region. The analysis process is the registration of those signal by a suitable analyzer.
An important issue is related to primary beam modifications induced on the sample. In general the input energy associated to the primary beam induced ef-fects as: chemical reactions, diffusion, recrystallization, morphological defor-mation.
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Serious limitations can be also associated to the environment required by the analysis. In fact many techniques required the use of an Ultra High Vacuum environment not suitable with all kinds of samples, as for examples bio and polymeric nanomaterials. Other limitation associated to the samples have, many times, to be considered as for examples, insulating characteristics, overall morp-hology, handling possibilities. In some case a large size sample is required.
Figure 1.1.1. Base scheme of characterization process
In this chapter the several analytical techniques used to characterize nanomate-rial are introduced. The techniques are divided in three main areas: Microscopy; Spectroscopy; Spectrometry.
The first one
Microscopy, gives information on sample morphology and it allows to determinate nanostructure, shape and size.
Spectroscopy allows to obtain composition analysis and chemical infor-mation.
Quantitative and depth profile characterization is done with
spectro-metry techniques.
In the following part of the introduction and an overview of many analytical techniques for nanomaterials are reported, in order to give an overall view.
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Characteristic parameter: Van der Waals force
Type of information: Surface topography and roughness; distribution of magnetic and electric domains; elasticity and viscosity of the surface. Lateral resolution: 2-5 nm (vertical resolution ~Å)
Environment: Air/vacuum/controlled atmosphere User skill level: High
Time request for a measurement: from 15 minutes to hours Cost equipment: Medium
1.1.2 Auger Electron Spectroscopy [AES]
Characteristic parameter: Electron energy spectrum
Type of information: Elemental composition; map analysis; depth profi-le
Lateral resolution: 30nm Sensitivity: 0.1 at%
Environment: UHV (Ultra-High Vacuum) User skill level: High
Time request for a measurement: 3 hours Cost equipment: Medium/high
1.1.3 Fourier-Transform Infrared Microscopy (FITR)
Characteristic parameter: molecular vibration
Type of information: Elemental and molecular distribution Lateral resolution: 5 microns
Environment: air User skill level: high
Time request for a measurement: 30 minutes Cost equipment: medium
1.1.4 Helium Ion Microscopy [HIM]
Characteristic parameter: emitted electron Type of information: morphology
Lateral resolution: 0.3 nm Environment: vacuum User skill level: medium/high
Time request for a measurement: 10 minutes Cost equipment: high
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1.1.5 Dynamic Secondary Ion Mass Spectrometry [SIMS]
Characteristic parameter: sputtered ions
Type of information: elemental composition; mass spectra, depth profi-le, line and map analysis
Lateral resolution: 0.1-10 microns Sensitivity: ppb-ppm
Environment: UHV(Ultra-High Vacuum) User skill level: High
Time request for a measurement: from 5 minutes to several hours Cost equipment: High
1.1.6 X-ray Fluorescence Analysis [XRF, EDX]
Characteristic parameter: Second X-ray Fluorescence Type of information: elemental composition
Lateral resolution: 100 nm Sensitivity: 0.1 %at Environment: air/vacuum User skill level: medium
Time request for a measurement: from few minutes to 1 hour Cost equipment: medium/low
1.1.7 Grazing-incidence X-ray Fluorescence
Characteristic parameter: characteristic emitted X-ray
Type of information: elemental composition; density; layer thickness Lateral resolution: 1 cm
Sensitivity: 10E12 at/cm2 Environment: air
User skill level: high
Time request for a measurement: 2 hours Cost equipment. Medium/high
1.1.8 Electron Backscatter Diffraction [EBSD]
Characteristic parameter: Electron diffraction and absorption
Type of information: Crystalline structure, orientation, strain, grains morphology and deformation.
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Time request for a measurement: hours Cost equipment. High
1.1.9 Scanning Electron Microscopy [SEM]
Characteristic parameter: distribution and energy of scattered and emit-ted electrons
Type of information: Topography Lateral resolution: 0.5 nm
Environment: Vacuum User skill level: Medium
Time request for a measurement: 5-10 minutes Cost equipment: medium/high
1.1.10 Scanning Tunneling Microscopy [STM]
Characteristic parameter: Spatial variation of electron tunneling current Type of information: map of surface electronic structure
Lateral resolution: 0.1 nm
Environment: UHV (Ultra-High Vacuum) User skill level: High
Time request for a measurement: 1 hour Cost equipment: high
1.1.11 Static Secondary Mass Ion Spectrometry [S-SIMS]
Characteristic parameter: sputtered atomic and molecular ions Type of information: Mass spectra; chemical image
Lateral resolution: 0.1 microns Sensitivity: 10E9 at/cm2
Environment: UHV (Ultra-High Vacuum) User skill level: high
Time request for a measurement: 10 minutes Cost equipment: high
1.1.12 Surface Raman Spectroscopy
Characteristic parameter: Optical emission Type of information: molecular absorption
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User skill level: high
Time request for a measurement: 10 minutes Cost equipment: medium
1.1.13 Transmission Electron Microscopy [TEM]
Characteristic parameter: electron scattering
Type of information: morphology; crystal structure; defect distribution Lateral resolution: 0.1 nm
Environment: UHV (Ultra-High Vacuum) User skill level: high
Time request for a measurement: 1 hour Cost equipment. High
1.1.14 X-ray Diffraction and Reflection [XRD]
Characteristic parameter: diffracted x-ray Type of information: surface crystal structure Lateral resolution: 0.1 mm
Environment: air
User skill level: medium
Time request for a measurement: 5-20 minutes Cost equipment: medium
1.1.15 X-ray Photoelectron Spectroscopy [XPS]
Characteristic parameter: photoelectron energy
Type of information: elemental composition; chemical bonding; nanola-yer thickness
Lateral resolution: 3 microns
Environment: UHV (Ultra-High Vacuum) User skill level: high
Time request for a measurement: hours Cost equipment: high
1.1.16 X-ray Reflectometry [XRR]
Characteristic parameter: X-ray intensity
Type of information: layer thickness, density, interface roughness Lateral resolution: 100nm
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User skill level: medium
Time request for a measurement: until several hours Cost equipment: medium
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SECTION 2
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The Scanning Electron Microscope is a widely used non-destructive surface analysis technique, which uses electrons both as a beam probe for surface investigation and as a signal for the generation of the microscopic surface image. The impinging electrons entering the sample are either elastic or inelastic scattered in a pear-shaped interaction volume. A fraction of electrons can escape from the surface and provide the topographic and compositional contrast information that will eventually compose the SEM image. The surface signal response is a fluence of electrons emitted in many directions from the target and collected by detectors positioned in various configurations. The detector is not a camera, it simply intercepts the electrons while the amplified electric signal is later on converted into a brightness level. The SEM image is the composition of a two-dimensional array of data, the sum of all consecutive brightness level spots corresponding to the intensity of the electrons expelled point by point from the specimen, collected by synchronizing the image pixels coordinates with the PE raster positions in the ROI (region of interest).
What determines the magnification power in SEM is simply the ratio between the fixed size of the screen where the surface image is displayed and the size of the PE rastering area on the sample. Therefore the smaller the raster and beam diameter, the bigger the magnification. The magnification power provided by SEM is much higher than the one that can be obtained by optical microscopes as electrons, with their wave-particle duality, can widely surpass the maximum resolution imposed by the visible wave-length diffraction limit. Nowadays SEM can reach five orders of magnification, from about 10x to 1millionx with 1nm of resolution, thanks to well established electron gun sources, electron optics and detectors.
The electron-matter interaction occurring in standard SEM analyses is manifold and the various phenomena that occur are useful for additional analytical techniques. The elastic scattering alters only the electron direction component, whereas the inelastic scattering can involve many different processes dissipating the electron kinetic energy in the target. The interaction volume occupied by all possible electron trajectories can be modelled through the Monte Carlo simulation. The signals generated can be SE, BSE, Auger electrons, characteristic X-rays, Bremsstrahlung and fluorescent X-rays, cathode-luminescence, and slight ESD. The characteristic signals originate in different spots within the interaction volume, have different escape depths and therefore
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provide different resolutions for the relative analyses. Besides the many correlated techniques, the topic of this paragraph will focus on SEM.
Acronyms
BSE Backscattered Electron CCF Cross Correlation Function CD Critical Dimension
CFE Cold Type Field Emission Electron Gun CL Chatodoluminescence
EBIC Electron beam induced current EBL Electron Beam Lithography EBSD Electron Backscatter Diffraction
EDX or EDS Energy dispersive (X-ray) spectroscopy EME electron mirroring effect
ESD electron stimulated desorption FEG Field Emission Gun
FFT fast Fourier transform
MIP molecularly imprinted nanoparticles PE Primary Electron
ROI Region of Interest SE Secondary Electron
SEM Scanning Electron Microscope SNR Signal to Noise Ratio
TFE Thermal Type (assisted) Field Emission Electron Gun UHV Ultra High Vacuum
WD Working Distance
WDS Wavelength Dispersive Spectroscopy
2.1.1 Instrumentation
It is worth noting that, in general, electronic images of adequate specimens can be obtained without complex electron optics thanks to the special electron-matter interactions. For example, low energy electron holography represents an outstanding recent demonstration that nanometer resolution microscopy can be achieved by exploiting a simple lense technique that uses a coherent low-energy electron source [1].
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A peculiar feature of SEM is the use of a focused PE beam as the probe and ROI rastering. The first implementation of a modern SEM can be found in Zworykin et al. in 1942 and its operating principles are still valid today. A basic SEM instrument consists of a PE column with the electrons source and electron optics operating in UHV condition, a sample stage that can often operate at lower vacuum levels and one or more detectors for SE and BSE [2].
The most simple sources of PE are cathodes heated at temperatures high enough to promote the thermionic emission of electrons. Alternatively, with FEG operating at cold temperature, electrons tunnel through the work function barrier and are emitted due to the strong electric field concentrated in a very sharp tip the size of few nanometers. FEG provides the highest brightness and lower energy spread, which translates into less chromatic aberrations, compared to other electron sources like hot tungsten cathodes with thermionic emission or lanthanum hexaboride LaB6. This difference is particularly significant at the very low electron energies, often useful to reduce charging effects on insulators. The drawback of FEG is its sensitivity to contamination from residual gas. To reduce contamination, UHV conditions are mandatory and recurrent brief heating (“flashes”) are applied to FEG to desorb gas molecules. Schottky FEG represents a compromise solution, where electron emission is thermal-assisted in order to achieve higher stability, with only small drops in performance.
The most common electron gun is an electrostatic lens composed of a cathode, maintained at negative potential, that is the source voltage which defines the PE energy, a Wehnelt maintained at slightly higher potential (i.e. more negative charge) and an anode plate at ground. The electrons exiting the electron gun focus in the first crossover and diverge immediately afterwards, thus further electromagnetic lenses and mechanical apertures are inserted in the column to finely direct the PE beam and control its shape on the specimen surface.
In SEM, it is desirable to have the highest beam collimation with the smallest spot diameter on the specimen, which provides the best resolution and possibly the enhanced sharpness level in the SEM image. In order to obtain high depth of focus and minimal aberrations, electromagnetic lenses typically operate in the column by increasing the convergence angle of the spiralling trajectories of electrons. A condenser lens controls the amount of PE current and an objective lens controls the final focus. Moreover, mechanical apertures can be interchanged in order to decrease the convergence angle and aberrations at the cost of the PE current. Scan coils guide the spiralling electron beam through the
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final objective lens and produce the raster on the specimen surface when the finally focused PEs exit the bottom of the pole piece.
The working distance, typically ranging from about 3 mm to 25 mm, determines the separation between the final lens and the specimen. Usually the stage that holds the sample is motorized with translational, rotational and tilt movements and in the case of a tilted surface a dynamic focus can automatically provide the necessary corrections on the PE alignment. The identification of the ROI is not the only purpose of the stage movement. The specimen orientation and the electrons take-off angle (related to the detectors) are critical factors that determine SE intensity and can greatly improve SEM image quality. The SEM image interpretation greatly depends on the detector location. The positioning of the detectors can be either in-chamber or in-column, with different outcomes. Electrons exiting the specimen with energies lower than 50 eV after inelastic scattering are classified as SEs, whereas BSEs have energies ranging from 50 eV to nearly the PE energy level. SEs coefficient
δ
and BSEs coefficientη
represent respectively the ratio of the number of SEs and the number of BSEs to the number of PEs. SEs can be further classified based on their generation mechanism.Type 1 SEs are expelled from the specimen in coincidence with the PE incidence spot, therefore they can provide high spatial resolution. Type 2 SEs are emitted after multiple BSEs scatterings within the specimen, at relatively high distance from the PE incidence spot for higher PE energies, therefore they produce either a lower resolution signal or a background signal. Type 3 SEs are generated when BSEs escape the specimen and hit the inner walls of the SEM chamber.
The SEM image contrast depends on both the surface morphology and the target materials. SEs emission varies with surface geometry and escape region. The term “edge effect” indicates that more SEs can escape when the PE beam hits steep surfaces, thus the edges appear brighter and provide the typical topographical appearance of SEM images. A noticeable consequence is that tilting the target can enhance the contrast. Our brain can easily read a surface topography when it is illuminated from above in visible light. Accordingly, in order to simulate a condition of natural illumination, the in-chamber SE detector is typically positioned at the top of the SEM image. This trick will allow the
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electron detector to mimic a light source illuminating the microscopic specimen from above.
In-lens detectors are also used. In this configuration, the detector is located inside the final lens and collects mainly the electrons emitted normal to the surface. Therefore it is particularly useful to collect electrons emitted also from deep cavities. In-lens detection can also minimize the artefacts induced by charging effects for non-conductive specimens. In-lens detectors equipped with energy filters can discriminate SEs and BSEs.
BSE detectors are positioned in the column axis and are more sensitive to the atomic number (Z) contrast, hence they can provide a qualitative discrimination among different elements. Electron backscattering against heavier elements is more efficient than for lighter elements, therefore the SEM image will appear brighter in correspondence of the higher Z numbers. The backscattered electrons have higher energy after elastic scattering and their escape depth can be a hundred times greater than that of SE.
Common types of detectors are designed based on the Everhart-Thornley configuration. Basically, this system consists of a scintillator plus a photomultiplier, with a Faraday cage operating as an energy filter that enables to discriminate BSEs from SEs [2]. Alternatively, in order to generate a signal, solid state detectors based on p-n junctions exploit the electron-holes pairs production in semiconductors when hit by electrons with suitable energy. This small electronic signal requires further current amplification.
The specimen current that flows through the bulk of the target hit by the PE can represent a detection signal as well.
Any detector measurement can be added to the acquisitions of other various analytical techniques to form a single combined map. For instance, topography and compositional images can merge, SE+BSE, EBSD+SE, EDX+EBSD or the specimen current signal and so on [3].
The most relevant factors that determine the performance level in a SEM are the beam diameter, the image resolution and sharpness. Besides the beam spot size and shape, the ultimate spatial resolution of an electron microscope depends on the interaction volume of the electron probe with the specimen. The PE maximum penetration depths can range from few nanometres, when hitting the bulk of elements with higher atomic numbers and for energies in the order of 1
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keV, to few micrometres for lighter elements and with PE energies of 20 keV. Typically, type 2 SEs can degrade image resolution, as they are generated far from the incident beam.
The beam shape can be altered by astigmatism or chromatic (i.e. energy) aberrations, whereas the interaction volume affects the SNR. Factors concurring to the SNR include the PE beam fluence, the statistical nature of the electrons collision with the sample and with the chamber walls, the take-off angle, the efficiency of the detector, and the electronic signal amplification and processing. Image pixel density is a fixed factor but it plays a role too. The maximum useful magnification can be reached only if the PE beam spot size is smaller than the pixel size, otherwise the image becomes blurred as the signal acquired within each pixel will see partial contribution from adjacent spots.
A frequent artefact that reduces sharpness is the target contamination that occurs when hydrocarbons molecules on the specimen surface or residual gases in the analysis chamber interact on the ROI hit by PEs.
2.1.2 Application Cases
SEM is a widely proven and versatile technique that is still today an indispensable metrology tool. Its overall simplicity in sample preparation is a remarkable advantage over other surface analysis techniques.
In CD metrology, an ultimate spatial resolution is pivotal and it is in the order of 1nm or less. Therefore, in most cases when SEM is applied to nano-materials, maintaining pristine surface conditions of the target at all times is essential. The resolution is mainly determined by the PE probe size and energy. In order to obtain the highest performance, cold FEG, with high brightness and less than 5nm source size, provides the high spatial and temporal coherency of the electron beam required to obtain the adequate probe diameters. The pear shaped interaction volume changes with the energy and the beam energy affects various parameters like the sampling depth of the backscattered electrons, the SEs coefficient, the charging effects. Other factors besides resolution become relevant for SEM imaging, as in the application cases described below.
The right balance between the signal level necessary to obtain an adequate SNR and the electron probe diameter must be found (Figures 2.1.1 and 2.1.2). As a rule of thumb, a smaller spot size provides higher resolution. Mechanical apertures are useful to reduce the spherical aberrations but they also cause the
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PE current to decrease. Also, diffraction effects can occur for smaller apertures diameters due to the wave nature of the electrons.
Periodic nano-ripples produced by gold ions implantation on Ge are reported in Figure 2.1.1. The size of the structures ranges from micrometer scale for the crest length to nanometer scale for the curled nanowires and the gold rich nanoparticles decorating the ripples. This hierarchy structure can play a relevant role in the cellular behaviour on nanostructured biocompatible scaffolds [4]. In the case of conductive materials, the SEM image has the best SNR when high PE energy is used to obtain a nanometer size PE diameter and when a low WD is selected to maximize the SEs collection with the detector in-lens.
Similar conditions are applied to ZnO nanoparticles doped with Au, as reported in Figure 2.1.2. These nanometer-scale powders are used in gas sensors [5]. In this SEM image Au nanoparticles decorating the bigger ZnO crystals are brighter and clearly visible, both due to the high gold BSE coefficient η and the considerable edge effect occurring on smaller particles. At very high magnifications, electromagnetic interferences or mechanical vibrations become more relevant in the final image and artefacts may show as wavy irregular edges. These artefacts do not depend on the electron optics but can be reduced only with the implementation of external noise insulating systems.
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Figure 2.1.2 ZnO nanoparticles doped with Au
In SEM metrology many materials are not conductive. Moreover, plastics and biological materials are affected by heat induced by the impinging PEs. MIPs are shown in Figure 2.1.3 as an example. These nanoparticles can enhance the
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surface plasmon resonance phenomena aiming to hormones detection [6]. In this case, low PE energies are preferable in order to preserve particle shapes. In general a satisfactory balance in secondary electrons yields can be reached, without the occurrence of local charging effects, by using low energies for PEs impinging on insulating particles. The nanoparticles should be deposited on a conductive substrate at ground potential. For instance, a simple chip of silicon wafer provides both good electron conductivity and a flat substrate with an even morphology.
Figure 2.1.3 MIPs nanoparticles dispersed on silicon substrate.
Specimen charging in electrical insulating materials can affect both SEM accuracy and reproducibility. At first, distortions and anomalous contrast can appear in the SEM image, but in extreme cases EME can occur [7], when the charge injected by electrons fluence in the target builds up to a level so high that the equipotential surface produced around the charged volume will elastically reflect back the PE beam. As a result the PE rastering will produce an image of the inner walls of the SEM chamber rather than of the ROI.
Sputter coating of ultrathin metal layers with thickness in the order of 1-5 nm can suppress the surface charging. Moreover high Z number conductive coatings, like Pd, Au or W, are useful to enhance the SE yield on low atomic number targets. When trying to achieve CD resolutions even very thin films can alter the appearance of the surface morphology. Therefore, alternative methods like the low PE energy “gentle beam” are necessary to obtain the charge balance without coatings. In this case, the PE beam can be delayed by applying a decelerating electric field to the target until it impinges on the specimen with energies as low as 100 eV. In more general cases, and for specific insulating materials and PE fluence, adequate PE energies can be selected to establish an equilibrium between the number of electrons injected in the interaction volume and the SEs plus BSEs escaping the surface. By the equilibrium conditions the total emitted electron coefficients will be 1.
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SiO2 is an example of common insulating material as seen in Figure 2.1.4. SEM
imaging of an inverse opal scaffold deposited on silica substrate is not straightforward. The pristine polystyrene nano-spheres used to deposit multilayer films dissolve after an annealing process and only the insulating opal framework remains. Local and not homogeneous charging effects can deflect the PE beam and distort the resulting SEM image.
Figure 2.1.4 Image of opal framework of SiO2
In the case reported in Figure 2.1.4, conductive coatings are not deposited [8]. However, PE low fluence and a rapid scan with the integration of many frames are necessary measures to avoid excessive surface charging. When the PE beam hits the inverse opal, some charge dissipation is promoted by the particular structure of the scaffold. The inverse opal configuration favours SE emission, therefore a charge balance condition between impinging and escaping electrons can be reached. Otherwise, EME is rapidly induced if the PEs hit the bulk of the silica substrate.
High energy PEs are less sensitive to deflections induced by local charging. Moreover, the reduced WD improves SEs collection in-lens. Moreover, the reduced WD improves SE collection by the in-lens. The in-lens BSE detector can collect the electrons emitted from deep cavities; in this way even the third
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level of pores is discernible in depth through the windows visible in the opal scaffold.
In the past, SEM image resolution was defined as the smallest width of the measurable particles or as the spacing between them. More recently, diffractograms were proposed as a way to determine resolution: they basically consist 2-dimensional representations of the SEM image spatial frequency by using fast Fourier transform FFT or by using the CCF cross correlation function. High resolution at high PE energies is mainly determined by type 1 SEs, when operating at high magnifications, since type 2 SEs emitted far from the PE incidence spot contribute only for a random noise. High resolution at low PE energies, i.e. at less than 5 keV or at about 1 keV, sees a much higher contribution of type 2 SEs since the interaction volume is reduced.
When SEM is applied to nanosheets (Figure 2.1.5), lateral resolution becomes less relevant, with respect to the ability to resolve different thickness appearing in greyscale levels. In general, surface details are obscured if the electron beam penetration is increased, i.e. for higher PE energies. In this case, the main goal of SEM imaging is to obtain a considerable enhancement in the slight contrast provided by each atomic layer. Here graphene oxide sheets deposited on silicon are shown. In the case of very thin sheets, relatively low PE energy provides good contrast on the layers when using the in-chamber detector positioned at intermediate take-off angle for SEs. In this condition, each superimposed carbon monolayer yields an area with a bit darker level by shielding a number of low energy SEs.
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Figure 2.1.5 Graphene sheets outstretched on silicon substrate
Apart from biological tissues, metals, geological specimens and many other kinds of samples which are not treated here, SEM is extensively used in many phases of semiconductor manufacturing, from production lines to device inspection, failure analysis or reverse engineering. Various metrology issues in CD-SEM are still open and need to be addressed when aiming at a standardization on the reference nano-materials, based also on other techniques like AFM. Chemical microanalysis or microstructural capabilities can be easily added to SEM by introducing in the specimen chamber complementary techniques such as EDX (EDS) or WDS and EBSD. Some other techniques strictly related to SEM, which are not discussed here but that should be mentioned, are chatodoluminescence CL, EBIC, magnetic contrast and EBL. In a process of multi-techniques cross comparison analysis in the nanomaterials field, we need to ensure the accuracy, reproducibility and traceability chain.
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References
[1] Low energy electron holographic imaging of individual tobacco mosaic virions, JeanNicolas Longchamp, Tatiana Latychevskaia, Conrad Escher, and HansWerner Fink, Applied Physics Letters 107, 133101 (2015)
[2] Scanning Electron Microscopy and XRay Microanalysis, J.I. Goldstein et al., Plenum Press New York.
[3] Advanced Scanning Electron Microscopy and XRay Microanalysis, Dale E. Newbury et al., Plenum Press New York.
[4] Rossana Dell'Anna, Cecilia Masciullo, Erica Iacob, Mario Barozzi, Damiano Giubertoni, Roman B¨ottger, Marco Cecchini and Giancarlo Pepponi; Multiscale structured germanium nanoripples as templates for bioactive surfaces, RSC Adv., 2017, 7, 9024–9030
[5] Gaiardo, A., Fabbri, B., Giberti, A., Guidi, V., Bellutti, P., Malagù, C., Valt, M., Pepponi, G., Gherardi, S., Zonta, G., Martucci, A., Sturaro, M., Landini, N. ZnO and Au/ZnO thin films: Room temperature chemoresistive properties for gas sensing applications (2016) Sensors and Actuators, B: Chemical, 237, pp. 10851094.
[6] Lucia Cenci, Erika Andreetto, Ambra Vestri, Michele Bovi, Mario Barozzi, Erica Iacob, Mirko Busato, Annalisa Castagna, Domenico Girelli and Alessandra Maria Bossi; Surface plasmon resonance based on molecularly imprinted nanoparticles for the picomolar detection of the iron regulating
hormone Hepcidin25. Journal: Journal of Nanobiotechnology. MS: 3155145631487099
[7] Clarke, D.R. & Stuart, P.R. (1970); An anomalous contrast effect in the Scanning Electron Microscope. J. Phys. E: Sci. Instrum. 3, 705707.
[8] Glass Micro- and Nanospheres: Physics and Applications. Giancarlo C. Righini, Ed. Pan Stanford, 2018.
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2.2 SCANNING PROBE MICROSCOPES (SPM)
Erica IACOB
[email protected]
FONDAZIONE BRUNO KESSLER
INTRODUCTION
Back in the early 1980s G. Binning and H. Rohrer dazzled the world with the first real-space atomic-scale images of surfaces. Their idea was to apply the tunnelling effect to a device/system in order to “see” the surfaces with atomic resolution. This discovery earned its inventors the Nobel Prize in Physics in 1986. Microscopy based on the tunnelling effect is called Scanning Tunnelling Microscopy (STM). STM is the ancestor of all scanning probe techniques (SPMs).
SPM is considered one of the modern powerful research techniques that allow capturing surface information such as morphology and other local properties in a relatively easy way. SPMs are used in a wide variety of disciplines, including fundamental surface science, routine surface roughness analysis, and spectacular three-dimensional imaging from atoms of silicon to micron-sized protrusions on the surface of a living cell. In some cases, scanning probe microscopes can measure physical properties such as surface conductivity, static charge distribution, localized friction, magnetic fields, and elastic moduli. Hence, SPM applications are very varied.
All SPM techniques are based on two fundamental components: the probe and the scanner. Probes can be described as needles (tip apex radius 5-10 nm) that scan the surfaces at a certain distance (0.1-10nm). Based on the various techniques, they can be made of tungsten, platinum-iridium, gold (STM), silicon (AFM), Ti or Pt coated silicon (SCM, SKM, SEM), Ni or Co magnetic coated silicon (MFM). When an SPM probe is placed in close proximity to the surface, the sensed interaction can be correlated to tip position and as the tip scans the surface, a 3d map is created. The positioning control of sample and/or tip depends on the scanner. All SPM scanners are based on piezoelectric ceramic material. Piezoeletric materials change their dimensions in function of applied voltage. This allow to control in a very precise way probe-sample distance and the position of the probe over the surface.
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Acronyms:
AFM Atomic Force Microscope EFM Electric Force Microscopy FWHM Full Width at Half Maximum LFM Lateral Force Microscopy SPM Scanning Probe Microscopes STM Scanning Tunnelling Microscopy SCM Scanning Capacitance Microscopy RMS Root Mean Square
2.2.1 Instrumentation
Scanning Tunnelling Microscopy (STM)
Scanning Tunnelling Microscopy (STM) provides information on the topography of a surface by measuring the tunnelling current occurring between the tip and the sample surface. This technique allows to measure conductive samples such as metals or semiconductors only, but it is very powerful as it can obtain true atomic resolution on some samples even at environment conditions. The instrument is based on a sharp conductive tip that scans the surface from a distance of only a few angstroms. The main STM techniques are “Constant Current” or “Constant Height” modes for "topographic" data acquisition. When bias voltage is applied between the tip and the sample, tunnelling current occurs. In Constant Current mode (CCM) the scanner keeps the current constant by feedback circuit. So vertical movement of the scanner (feedback signal) reflects surface topography. On the contrary, in Constant Height mode (CHM) the scanner of the STM moves the tip horizontally only, hence current between the tip and the sample surface varies according the sample relief. With this mode a higher speed can be obtained as feedback on tip height is not necessary. However, CHM can only be applied if the sample surface is very flat, since surface corrugations higher than 5-10 Å can seriously damage the tip. This technique can be applied to conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates.
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Atomic Force Microscopy (AFM)
In Atomic Force Microscopy the sample is probed by a silicon tip. This stylus, with a tip apex often less than 10 nm, is mounted on the free apex of a silicon cantilever that is usually 80 to 300 micron long. The red spot of a laser diode is reflected from the backside of the cantilever (Figure 2.2.1)). Tip position is obtained/acquired by the position of the laser diode red light reflected by the cantilever on a photodiode screen. When scanning, the feedback system minimizes the deflection by adjusting the vertical position of the sample. The AFM lateral resolution is determined by the tip apex dimension and by the sensitivity in detecting the spot laser position on the photodiode.
The main force occuring between tip atoms and sample atoms is an interatomic force called van der Waals force. Depending on the sample-tip distance, two measuring modes are possible: in the contact method, the tip slides very close (a few angstroms) from the surface, originating the repulsive interatomic force. In the non-contact method, the cantilever is held tens to hundreds of angstroms from the sample surface and the interatomic force produces the attraction [Garcia et. all, 2002]. In addition to the van der Waals force, other forces occur. For instance, in the contact mode the capillary force plays a critical role since the thin water layer that is often present in the environment holds the tip attached to the sample surface. In absence of external field the dominant forces are van der Waals interactions, short-range repulsive interactions and long range adhesion forces but also capillary forces and elastic cantilever forces. In short, the distance regime (i.e., the tip-sample spacing) determines the type of force that will be sensed.
The contact mode is preferred when atomic scale images are needed as in this mode the tip is in close contact with the sample and a better lateral resolution can be achieved. Since a strong mechanical interaction occurs between the tip and the sample surface, the contact mode is suitable for hard and relatively flat surfaces but not appropriate for soft samples such as organic or biological objects.
In the non contact mode, the system forces the tip to vibrate (close to the cantilever resonance frequency) near the sample surface at a distance between tens to hundreds of Angstrom. Vibrating scanning modes include the noncontact mode, the intermittent-contact mode, the oscillatory technique, the semi-contact mode, the tapping mode, etc. They differ in the distance at which the tip is kept
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from the surface and in the tip oscillation feedback control. The limited contact with the surface reduces tip wear and tear and surface damage. For this reason, this technique is suitable for any kind of sample, from soft to hard in a wide range of sample topography.
Figure 2.2.1. Schematic view of the Atomic Force Microscopy technique.
Artifacts and Resolution: Since AFM is a “contact technique”, many factors
can affect image resolution. The two main ones are tip interaction and scanner properties.
Figure 2.2.2. Graphic representation of tip-sample scanning resolution. If we consider tip influence, it is a fact that the final picture (profile scan) is a convolution of tip apex size, cone angle and dimension of surface morphology [Eaton et. al, (2010)]. In Figure 2.2.2 it is shown how tip cone angle can affect
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final AFM image resolution: A) a tip scans a sphere attached on a surface. Tip apex dimensions and tip cone angle cause a broadening of the measured profile, compared to the real sphere dimension. B) and C) show the scan profile of two sphere attached to a flat surface: the scans are performed by two tips with different cone angles. In B) the sharp tip can resolve the two shapes, in C) the two spheres are not resolved by the dull probe.
Moreover, any unpredictable damage of the tip apex (double/multiple tip, a fractured tip, particles attached from surface causing a dirty/contaminated tip, blunt tip) can cause morphological artefacts on final AFM image (Figure 2.2.3).
Figure 2.2.3. Artefacts due to a broken tip.
Many other factors can interfere with the scanning operation. They are due to peculiarities of the piezoelectric scanner and are: creep, hysteresis and scanner drift (that can cause image distortion), and edge overshoot (that can cause an increased measurement of step height).
It is also worth mentioning other causes of artefacts unrelated to tip and scanners. We can list, for instance, background bow/tilt, due to the intrinsic curved motion of the probe during scanning operations (frequent) or the intrinsic tilt due to sample mounting. Both this artefacts can be corrected with a 1st and 2nd order plane subtraction.
Any of the above artefacts can occur on a daily basis. They must be identified and removed as better as possible by electronic correction, post processing software correction, tip changing, sample grounding, etc. Experience can provide guidance in finding the best solution.
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Electric Force Microscopy (EFM)
In Electric Force Microscopy (EFM) the probe, a metal-coated silicon tip, can ‘feel’ some electric properties of the surface [Stangoni, 2005; Girard 2001]. In main configuration, a grounded sample is scanned by a DC biased cantilever. The opposite operation is also possible: a DC biased sample is scanned by a grounded cantilever. In this way, it is possible to obtain both a topographic image and a spatial distribution of the electric forces.
The EFM measurement is obtained either with a single scan or with the so called “two-pass technique”. The first method allows to obtain both topographic and electric information in a single scan, while, in the “two pass technique” the measurement is performed in two phases. In the first scan, performed in contact mode, the tip acquires the surface morphology, then, the tip is raised at a constant distance from the surface (10-100 nm) and the EFM measurent is performed. The “two-pass technique” allows to exclude the topographic influence during measurements and reduces tip damage that could be caused, for example, when removing the conductive coating layer from tip apex. This method enables to study the conductivity and electric pattern of sample surfaces, such as semiconductor devices and composite conductors.
Scanning Capacitance Microscopy (SCM)
Scanning Capacitance Microscopy (SCM) is another technique that allows collecting electric information on material surfaces. SCM images sample capacitance distribution. A metal-coated tip is needed in this technique. The measurement is performed with the “two-pass technique”. During the first pass, the tip - in semi-contact mode - collects information on the topography of the sample surface; in the second pass, the tip operates in tip-sample constant height mode. A time-varying bias voltage is applied between the metal-coated tip and the sample. As the probe-sample separation is kept constant, the variation in tip vibration amplitude is related to variation in probe-sample capacitance. The scan is performed all over the selected area and the resulting variation in capacitance is mapped. This technique is widely applied in the semiconductor industry. Many applications like dopant distribution maps, failure analysis, variations in the thickness of a dielectric material on a semiconductor substrate, sub-surface charge- carrier distributions can be obtained [Stangoni, 2005; Girard 2001].
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Kelvin Probe Force Microscopy (KPFM) or Scanning
Sur-face Potential Microscopy (SSPM)
Scanning surface potential microscopy (SSPM), also known as Kelvin Probe Force Microscopy (KPFM) is a method used to obtain information on surface potential distribution. The electrical signal is acquired by using a metal-coated silicon tip. The scanning is performed with the “two-pass technique”. During the first pass, the tip acquires the topography of the sample surface in semi contact mode (mechanically excited at its resonant frequency), in the second pass the tip is raised at a fixed distance from the sample, the tip is electrically excited at its resonant frequency and a DC bias and an AC component is applied to the cantilever. The DC component is adjusted in order to nullify the oscillation amplitude of the tip. When this condition is satisfied, this means that the DC component equals the local surface potential. By scanning the sample surface it is possible to obtain its potential map.
KPFM allows to obtain information on the electrical properties of metallic nanostructures. Moreover, high-resolution KPFM has been used to probe semiconductor devices in order to provide high-resolution potential profiles [Wilhelm et. al, 2011] as well as to investigate electronic properties of defects on clean semiconductor surfaces.
Magnetic Force Microscopy (MFM)
This technique can map the magnetic domain in magnetic materials. It usually requires silicon or silicon nitride tips coated with a thin magnetic film of Co or CoCr. In order to minimize topographic influence, the measurement is performed with the “two-pass technique”. After the acquisition of the surface profile in the first scan, the tip is raised at a fixed distance from the sample and moved over the surface following the surface topography contour. If the distance is “big enough” the tip is not affected by surface topography influence but “feels” just long range forces such as, in this case, the magnetic forces of the sample. The controller registers the amplitude and the phase variation of the cantilever oscillation that depends on the spatial variation in the magnetic field. MFM allows to observe magnetic domains whose sizes vary from several to several tens of nanometres. Applications range from studies of magnetism in rocks to magnetic material inclusions, MFM of hard disks, etc.
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Lateral Force Microscopy (LFM)
When the tip slides on a sample surface, Lateral Forces are generated. They can be considered drawbacks if the study aims to obtain topographic information. Moreover, if the sample is soft, the tip, scanning in contact mode can scratch the surface and can collect adsorbates particles. On the contrary, if the sample is hard, like silicon or metal, the tip can slips modifying image resolution or introducing some artefacts. Hovewer the torsion motion of the cantilever can be used to collect information on changes in chemical composition of surfaces. In lateral force mode (or torsion mode, or frictional mode) the system records information on the forces exerted upon the probe tip in the lateral direction as it scans across a surface. This information is collected in contact mode, together with topography. If a surface is perfectly flat, variation in the phase signal can provide information on changes in composition or on variation in frictional forces. It is also possible to provide quantitative information on friction values if tip and cantilever dimensions, as well as cantilever spring constant are known. This method can be applied to many different materials such as semiconductors, polymers, thin film layers, data storage devices in order to study surface contamination, chemical speciation and frictional characteristics in the nanotribology field.
2.2.2 Application cases
As explained in the previous section, AFM is a versatile technique that finds application in different areas. AFM can help in metallurgy to determine final production surface aspects. It can be applied to biologic samples, for its possibility to investigate cells and molecules in liquid and physiological solution, or can be applied to microelectronic materials to investigate morphology but it can also give information on surface conductivity or dopant active areas. The example below gives an idea of some of its applications.
Determining nanoparticles size
In this example, acrylamide based nanoparticles (NPs) were used to target the hormone Hepcidin-25 that can give information on iron dis-metabolism and doping. These particles were produced by precipitation polymerization and a post-production size characterization was required. Since the particles were provided in a high-density aqueous solution, sample preparation was the bottleneck of this study case.
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First, the authors deposited a drop of NPs solution on a silicon substrate and dried it in vacuum. Figure 2.2.4.A shows the area covered by the substrate. NPs are deposited in a continuous layer and due to their density, determining their dimension (height and FWHM) from the cross section is not reliable. The authors decided to dilute the solution. After two successive 1:10 dilutions, the deposited particles appeared isolated (figure 4B) and cross section analysis can give the correct diameter and height information (a statistical analysis was performed) [Cenci et. al, 2015].
The AFM images were acquired with a Solver Px Scanning Probe Microscope from NT-MDT. AFM data were acquired in semi-contact mode with a silicon tip (~5.5 N/m, ~120 KHz) with a nominal radius of less than 10 nm. Analyses were performed with a scanning areas of 1x1µm2.
Figure 2.2.4. AFM images of nanoparticles attached to a flat silicon surface. A) as received, high density. On the right, the cross section of the white line across some nanoparticles. B) dispersed after 1:100 dilution. On the left, a zoomed area. On the right a 1x1 micron scan area, z range in the vertical bar.
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Stainless steel
Topographical studies can often provide information about other bulk properties such as hardness or friction. In particular, surface roughness can affect the measurement of hardness. Coatings based on amorphous carbon, the so called diamond-like carbons (DLC) coatings, are widely used to increase hardness in the original material. Moreover, DLC coatings allow to increase wear resistance in components subjected to severe working conditions, have good corrosion resistance and high biocompatibility. Topographic analysis at the micro- or nano-scales are essential for functional thin coatings characterization [Borrero et al, 2010]. In this example, the required analysis was the morphological characterization of steel surfaces coated with DLC prepared in various conditions. The work [Onorati et al., 2017], in fact, was dedicated to presenting an original alternative method to evaluate nano-hardness other than the conventional use of micro indenter. The possibility to evaluate coating hardness was tested by using ion beam sputtering through Secondary Ion Mass Spectrometry (SIMS).
Figure 2.2.5. AFM images, 10x10 scan areas. Image A) Pristine widia surface (RMS=9 nm), images from B to D show the same surface with three DLC coatings [Onorati et al., 2017]. RMS is 155.6 nm, 95.6nm and 20.5 nm respectively.
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Figure 2.2.5.A shows the original substrate (widia, a very hard material usually used in cutting tools and other industrial applications) while figures 5B, C, D show three different DLC coatings. The morphology of the surfaces of the different coatings was characterized with an Atomic Force Microscopy (AFM) instrument from NT-MDT (UniSolver). Analysis was performed in semi-contact mode with a silicon tip with a nominal radius of less than 10 nm. Sample scans were performed in different positions with 20x20, 10x10, 5x5 and 1x1 μm2 scan size. For each scan area acquired, we measured the average surface roughness (Sa) and the root mean square surface roughness (Sq) [UNI EN ISO 4287, UNI EN ISO 4288].
Electrical measurements
Sometimes materials are important not only for their nanoscale structure but for other physical properties. Graphene based coatings are becoming very appealing lately since graphene properties can substantially modify bulk properties of sublayer materials. One of the primary reasons for the interest in graphene materials is its impressive electrical properties but also mechanical properties such as high strength and hardness as well as low friction. The example in Figure 2.2.6.A shows an AFM height image (500x500 nm2) of silicon periodic
ripples. Structures show a period of 20-30 nm and a ripple height of 2-3 nm. The image to the right, Figure 2.2.6.B, shows the SKM image of the same area. No surface potential signal is obtained.
Figure 2.2.6. A) AFM image on nanostructured silicon surface, B) SKM image on the same surface. 500x500 nm2.
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Figure 2.2.7 shows the same sample with a graphene monolayer deposited by CVD. The image on the left was performed in semi-contact mode, the one on the right was acquired in SKM mode. In this case, where graphene layer was present, a surface potential signal was generated.
Analysis was performed with a Solver Px Scanning Probe Microscope from NT-MDT. AFM data were acquired with a Pt coated silicon tip (~11.8 N/m, ~240 KHz) with a nominal radius of 35 nm. First pass height scan was performed in semi-contact mode. In the second pass, surface potential data were acquired rising the tip at 10nm from surface profile, in SKM mode.
Figure 2.2.7. A) AFM image on Graphene on nanostructured silicon surface, B) SKM image on the same surface. 500x500 nm2.
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Conclusions
STM is a very powerful technique that allows a rapid and relatively inexpensive investigation of sample morphology on the atomic-nano scales. However, this technique can be applied only to conductive samples, and often requires a vacuum environment and an active isolation from external ambient vibration. On the contrary, AFM is a versatile technique that is suitable for any kind of sample with morphology roughness in the range of a few microns. In fact, AFM allows to visualize, in 3D image, features in the range of few nanometer size including atoms and molecules on a surface. In the recent years many applications have been developed to measure other surface properties together with morphology by varying tip coating and feedback control. In addition to physical dimension, it is possible to analyse: hardness, friction, electrical or magnetic signal; and also to manipulate (move) object across the surface.
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References
[Mironov, 2004] Mironov V. L. (2004), Fundamentals of scanning probe micros-copy, the Russian academy of sciences institute, Retrieved from http://ip-ras.ru/UserFiles/publications/mironov/Fundamentals_SPM.pdf.
[Garcia, 2002] Garcia R. and Perez R. (2002), Dynamic atomic force microscopy methods, Surface science reports 47, 197-301.
[Bowen, (2009)] Hilal N. (2009), Atomic Force Microscopy in process engineering. An Introduction to AFM for Improved Processes and Products. Hardcover ISBN: 9781856175173 eBook ISBN: 9780080949574.
[Stangoni, 2005] Stangoni M. V. (2005), Scanning Probe Techniques for Dopant Profile Characterization, Diss. ETH No. 16024, retrieved from http://e-collec-tion.library.ethz.ch/eserv/eth:28140/eth-28140-02.pdf.
[Girard, 2001] Girard P. (2001), Electrostatic force microscopy: principles and some applications to semiconductors, Nanotechnology 12(4), 485-490.
[Wilhelm, 2011] Melitz W., Shen J., Kummel A. C., Lee S., (2011), Kelvin probe force microscopy and its application, Surface Science Reports 66, 1–27. [Eaton, 2010] Eaton P. and West P. (2010), Atomic Force Microscopy, ISBN:9780199570454.
[Cenci, 2015] Cenci L., Andreetto E., Vestri A., Bovi M., Barozzi M., Iacob E., Busato M., Castagna A., Girelli D. and Bossi A. M., (2015). Surface plasmon resonance based on molecularly imprinted nanoparticles for the picomolar detection of the iron regulating hormone Hepcidin-25, J. Nanobiotechnol 13:51 DOI 10.1186/s12951-015-0115-3.
[Onorati, 2017] Onorati E., Iacob E., Bartali R., Barozzi M., Gennaro S., Bersani M., (2017) Experimental study by Secondary Ion Mass Spectrometry focused on the relationship between hardness and sputtering rate in hard coatings, Thin Solid Films 625, 35–41.
[UNI EN ISO 4287] UNI EN ISO 4287, Geometrical Product Specifications (GPS) -- Surface texture: Profile method -- Terms, definitions and surface texture parameters.
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[UNI EN ISO 4288] UNI EN ISO 4288, Geometrical Product Specifications (GPS) -- Surface texture: Profile method -- Rules and procedures for the assessment of surface texture.
[Borrero, 2010] Borrero-Lopez O., Hoffman M., Bendavid A., Martin P. J. (2010), Substrate effects on the mechanical properties and contact damage of diamond-like carbon thin films, Diamond and Related Materials, 19 1273–1280. [Iacob, 2016] Iacob E., Dell'Anna R., Giubertoni D., Demenev E., Secchi M., Böttger R., Pepponi G., (2016) Nanofabrication of self-organized periodic ripples by ion beam sputtering, Microel Eng, 155, 50–54.
